1. Field of the Invention
This invention relates generally to relatively high speed, high density information storage and retrieval mechanisms utilizing a storage medium carried on a rigid disk, and more particularly to an improved read/write positioning mechanism for an open-loop positioning of the read/write head of such a device.
2. Description of the Prior Art
Amongst the electromechanical devices utilized for auxiliary memory and data storage, the so-called Winchester Drive provides the more advanced and economical means for data storage. Fundamentally, Winchester Drives utilize a spinning disk, typically an aluminum disk having a magnetic oxide coating thereon, which stores data as provided by a read/write head in concentric, closely spaced rings or tracks on the disk. Typically the read/write heads are carried on access arms and positioned adjacent to both sides of the spinning disk with the head induced to "fly" adjacent the disk as a result of an air layer carried by the spinning disk. In order to provide the high density of data in the oxide coating it is necessary that the read/write head be spaced closely adjacent the oxide coating, i.e., on the order of 1/2 to 1 microns, and that closely packed concentric tracks of data be recorded on the spinning disk. To accomplish this latter objective, it is necessary that the read/write head be positioned extremely accurately along the radius of the spinning disk to facilitate such close spacing of the recorded data.
Positioning of the read/write heads relative to the spinning disk is conventionally accomplished according to two broad servo system approaches, i.e., closed-loop and open-loop systems. In the more involved and expensive closed loop system, reference positions, either dedicated or embedded in the recorded data, are provided on the disk itself such that the servo positioning of the read/write head may be accurately accomplished by search for and recognition of reference matter recorded upon the disk medium or transducer means, separate from the recording disks--such as optical, inductive, or capactive transducers mounted to the actuator. However, in the latter instance, i.e., open-loop systems, the read/write head is positioned relative to the spinning disk by, in a typical instance, indexing a stepping motor to a predetermined position such that the read/write head is reproducibly positioned adjacent the selected concentric track of data. Since the concentric data tracks are typically spaced 2 to 4 thousandths of an inch apart, it will be recognized that rapid and accurate reproducible positioning of the read/write head across a disk which may be typically 51/4 inches in diameter is an extremely demanding undertaking.
An example of a positioning mechanism adapted for use in an open-loop system, such as a Winchester Drive, is to be found in U.S. Pat. No. 4,161,004, issued July 10, 1979. This patent discusses a capstan drive around which a flexible band is positioned with the band attached to a carriage mechanism. As a motor rotates the capstan, the band drives the carriage to a preselected position. Though the patented drive mechanism has proven economical and effective, certain shortcomings have also been noted. The structure of the band-capstan interface requires that the band be attached to the capstan thereby permitting but one rotation of the stepping motor attached to the capstan before the limited travel of the band is reached. Accordingly, the resolution of the head position is limited by the incremental movement of the stepping motor, or influenced by the cost of a stepping motor of very small incremental movement. However, given the great demands of an open-loop system in which the dimensions between the stepping motor and the read/write head must be precisely maintained under varying conditions and in the face of wear, other seemingly simple mechanical means, such as the lead screw means discussed in U.S. Pat No. 3,814,441 also suffer from various drawbacks. In addition to the need for accurate and reproducible positioning, the head must also move with great acceleration and deceleration to the desired track. Accordingly, inertial forces must be minimized, and movement dampened rapidly to permit the head to assume a steady position over the desired track. However, motion dampers add mass thus requiring yet another compromise in the known designs.